Wake County Sheriff Donnie Harrison said investigators were seeking cellphone video of the incident on the Vortex ride, WNCN-TV in Raleigh reported.
"The ride had stopped and they were fixing to offload when it started off again. That is preliminary," Harrison said.
Harrison said the injured ranged in age from 14 to 39, and some are related. One was a ride operator.
Fairgoer Caleb Norris told WNCN he turned around when he heard a loud crashing sound. He said he saw two people lying on the ground and a ride operator on his knees crying.
The ride, manufactured by an Italian company, Technical Park International, made its first appearance at the fair this year.
Brian Long, a State Fair official, said the state Department of Labor and the sheriff's department will investigate the incident. He said the fair will be open Friday.